Process monitor for monitoring an integrated circuit chip

ABSTRACT

A system or apparatus for monitoring an Integrated Circuit (IC) chip includes: a sense circuit at least partially constructed on the IC chip and configured to produce one or more sense signals each indicative of a corresponding process-dependent circuit parameter of the IC chip; and a digitizer module configured to produce, responsive to the one or more sense signals, one or more digitized signals each representative of a corresponding one of the sense signals. A controller is configured to determine a value of one or more of the process-dependent circuit parameters based on one or more of the digitized signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. application Ser. No.10/440,311, titled, “Process Monitor for Monitoring an IntegratedCircuit Chip,” filed May 19, 2003, which claims priority to U.S.Provisional Application No. 60/430,061, titled “Amplifier Assembly withAGC for a Tuner,” filed Dec. 2, 2002, which are incorporated herein byreference in their entireties.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to monitoring anintegrated circuit (IC) chip.

[0004] 2. Related Art

[0005] Circuits constructed on an IC chip or substrate are referred toas integrated circuits. Integrated circuits include transistors andresistors, for example. Integrated circuits are fabricated ormanufactured in high volume using integrated circuit processes, such asa CMOS process. The integrated circuits may be characterized in terms ofvarious circuit parameters, such as sheet-rho, transistor thresholdvoltage, and a transistor transconductance parameter, to name but a few.

[0006] Process variations can cause unpredictable and undesiredvariations of the circuit parameters, which can adversely affect circuitperformance. In other words, the circuit parameters tend to be processdependent. Thus, it is useful for a manufacturer to be able to quantifyor determine the circuit parameters. Accordingly, there is a need to beable to measure and determine process-dependent circuit parametersassociated with circuits constructed on an IC chip. A related need is tobe able to determine a temperature of the IC chip and/or a power supplyvoltage of the IC chip.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to methods and apparatuses formonitoring an IC chip. The invention provides a way of determiningvarious important circuit parameters, environmental parameters of the ICchip (such as temperature), and/or operational conditions (such as powersupply voltage) of the IC chip. The circuit parameters includeprocess-dependent circuit parameters. The invention can determineabsolute values of the circuit parameters. From this, the performance ofcircuits constructed on the IC chip can be evaluated.

[0008] An embodiment of the present invention is directed to a sensecircuit used in monitoring an IC chip. The sense circuit includes one ormore sense elements constructed on the IC chip. The sense elementsinclude diode connected transistors (both NMOS and PMOS) of differentsizes and operating currents, and on-chip resistors of different valuesand types. The sense circuit also includes a switch circuit or matrixthat provides different values and types of currents to each of the oneor more sense elements. The voltage developed across each of the senseelements provides useful information about how the IC chip wasprocessed.

[0009] In a further embodiment, the sense circuit feeds the varioussense voltages to a multiplexer (MUX), which presents selected ones ofthe sense voltages to a digitizer, such as an Analog-to-Digitalconverter (A/D). The A/D converts each analog voltage to a digital word.Together, the multiplexer and digitizer form a digitizer module. Aprocessor or controller, which may be external to the IC chip, controlsthe sense circuit and the digitizer module, and processes the varioussense data (digital words) produced thereby. The processor determinesvarious circuit parameters of the IC chip based on the digital words.

[0010] In an embodiment, a system for monitoring an Integrated Circuit(IC) chip, comprises: a sense circuit at least partially constructed onthe IC chip and configured to produce one or more sense signals eachindicative of a corresponding process-dependent circuit parameter of theIC chip; a digitizer module configured to produce, responsive to the oneor more sense signals, one or more digitized signals each representativeof a corresponding one of the sense signals. In a further embodiment, acontroller determines a value of one or more of the process-dependentcircuit parameters based on one or more of the digitized signals. Ineven further embodiments, the sense circuit and digitizer producesignals indicative of a temperature and a power supply voltage of the ICchip.

[0011] Further embodiments, features, and advantages of the presentinvention, as well as the structure and operation of the variousembodiments of the present invention, are described in detail below withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0012] The accompanying drawings, which are incorporated herein and forma part of the specification, illustrate the present invention and,together with the description, further serve to explain the principlesof the invention and to enable a person skilled in the pertinent art tomake and use the invention. In the drawings, like reference numbersindicate identical or functionally similar elements.

[0013]FIG. 1 is a block diagram of an example system including a processmonitor, according to an embodiment of the present invention.

[0014]FIG. 1A is a high-level block diagram of an example processmonitor system corresponding to FIG. 1.

[0015]FIG. 2 is a circuit diagram of an example arrangement of a biascircuit and a process monitor sense circuit of the process monitor ofFIG. 1.

[0016]FIG. 2A is a circuit diagram of an example power supply senseelement used in the process monitor sense circuit of FIG. 1.

[0017]FIG. 2B is a circuit diagram of an example “nominal voltage”monitor used in the process monitor sense circuit of FIG. 1.

[0018]FIG. 3 is a circuit diagram expanding on the bias circuit of FIG.2.

[0019]FIG. 4 is a circuit diagram of an arrangement of a bandgap voltagereference circuit.

[0020]FIG. 5 is a block diagram of a simplified arrangement of a processmonitor, according to an embodiment of the present invention.

[0021]FIG. 6 is a circuit diagram of an exemplary amplifier ofoperational circuit of FIG. 1.

[0022]FIG. 7 is a flowchart of an example method that may be performedby the process monitor of FIG. 1.

[0023]FIG. 8 is a flowchart of further method steps expanding on themethod of FIG. 7.

[0024]FIG. 9 is a flowchart of further method steps expanding on themethod of FIG. 7.

[0025]FIG. 10 is a block diagram of an example controller architecturefor performing various functions and methods associated with embodimentsof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] I. Glossary

[0027] CTAT: Constant-To-Absolute-Temperature.

[0028] CMOS: Complimentary Metal Oxide Semiconductor.

[0029] ADC or A/D: Analog-to-Digital Converter.

[0030] DAC or D/A: Digital-to-Analog Converter.

[0031] FET: Field Effect Transistor, having a gate terminal (gate), adrain terminal (drain), and a source terminal (source).

[0032] IC: Integrated Circuit.

[0033] NMOS transistor: N-channel MOS transistor.

[0034] PMOS transistor: P-channel MOS transistor (complementary to anN-channel transistor).

[0035] PTAT: Proportional-To-Absolute-Temperature.

[0036] VT, vt, Vt, VTH or VTH: Represents a gate-to-source (or“gate-source”) threshold voltage of a transistor.

[0037] Sheet-rho: Sheet Resistance, in units of ohms per square(ohms/square). Resistors may be constructed on an integrated circuitusing resistance material, such as polysilicon, configured in multiplesof a “unit square” of the material. Typically, a designer may have achoice of constructing a resistor out of a material having low sheet-rho(e.g., 6 ohms/square) or high sheet-rho (e.g., 200 ohms/square). Forexample, a resistor may be made using a polysilicon high sheet-rhomaterial. This is referred to as poly-high sheet-rho material.Alternatively, the resistor may be made from a polysilicon low sheet-rhomaterial, referred to as poly-low sheet-rho material. The sheet-rho of amaterial is related to its resistivity. Resistors can also be made frommetal, or semiconductor diffusions, for example.

[0038] II. Process Monitor System—Overview

[0039]FIG. 1 is a block diagram of a system 100 in which embodiments ofthe present invention can operate. System 100 includes a process monitorsystem 102 (indicated in dashed-line) for monitoring process-dependentcircuit parameters, environmental conditions (such as temperature) of anIC chip or substrate, and/or operational conditions (such as a powersupply voltage) of the IC chip. The circuit parameters are considered“process-dependent” because they depend on the particular technologyused to process the IC chip. Also, the circuit parameters may vary fromIC chip to IC chip, for any given design, because of manufacturingvariations in a given process across different IC chips. IC chip 103 maybe a monolithic integrated circuit using a CMOS process. IC chip 103 mayuse other types of technologies or processes, such as Silicon Bipolar,Gallium-Arsenide (GaAs), Indium Phosphide (InP), or Silicon Germanium(SiGe) processes.

[0040] Process monitor system 102 (also referred to herein as processmonitor 102) includes a bias circuit 104, a process monitor sensecircuit 106, a multiplexer 108, an optional scaling amplifier 110, adigitizer or Analog-to-Digital converter 112, and a controller 114.System 100 also includes an operational circuit 116 constructed on ICchip 103.

[0041] Bias circuit 104 includes one or more current generators forgenerating one or more bias currents 118, including bias currents I1,I2, I3 and I4. Controller 114 provides a circuit configuration controlsignal 120 to operational circuit 116. Controller 114 generally alsoprovides a clock signal 122 to digitizer 112, multiple select signals124 to multiplexer 108, and multiple select signals 126 to sense circuit106. In an embodiment, the bias circuit can include a bandgap voltagereference.

[0042] Sense circuit 106 is at least partially constructed on IC chip103. Responsive to bias currents 118 and select signals 126, sensecircuit 106 produces one or more sense signals 130 indicative ofcorresponding process-dependent circuit parameters of IC chip 103, atemperature of the IC chip, and a power supply voltage (VDD) of the ICchip. Sense signals 130 are indicative of circuit parameters of circuitsin sense circuit 106, and also of other circuits of IC chip 103 that areconstructed using the same process as the sense circuit, assumingrelatively uniform process variations and device physical orientationsacross the IC chip. Sense circuit 106 produces the following sensesignals:

[0043] a sense signal 132 (ph_rho) indicative of a high sheet-rho of ICchip 103;

[0044] a sense signal 134 (pl_rho) indicative of a low sheet-rho of ICchip 103;

[0045] a sense signal 136 (pmos_vt) indicative of a gate-to-sourcethreshold voltage of a PMOS transistor constructed on IC chip 103;

[0046] a sense signal 138 (nmos_vt) indicate of a gate-to-sourcethreshold voltage of an NMOS transistor constructed on IC chip 103;

[0047] a sense signal 140 (pmos_k) indicative of a transconductanceparameter (K) of a PMOS transistor constructed on IC chip 103;

[0048] a sense signal 142 (nmos_k) indicative of a transconductanceparameter (K) of an NMOS transistor constructed on IC chip 103;

[0049] a sense signal 144 (therm) indicative of a temperature of IC chip103; and

[0050] a sense signal 145 (vdd_sense or VDD_SENSE) indicative of a powersupply voltage (VDD) of the IC chip 103.

[0051] In different arrangements, sense circuit 106 may produce more orless sense signals than those listed above, and also, sense signals thatare different from those listed above. In an arrangement, each of sensesignals 132-145 is an analog voltage, however, in another arrangement,each of the sense signals may be an analog sense current. Alternatively,sense signals 132-145 may include both analog sense voltages and analogsense currents. Also, sense signals 132-142 (indicated generally at 146)are indicative of process-dependent circuit parameters of IC chip 103,while sense signal 144 is indicative of temperature and sense signal 145is indicative of power supply voltage.

[0052] Sense circuit 106 provides sense signals 130 to respective inputsof multiplexer 108.

[0053] At any given time, multiplexer 108 routes a selected one of sensesignals 130 to an output node 150 of multiplexer 108, in accordance withswitch control signal 124. Thus, multiplexer 108 passes a selected sensesignal 152 to an input of scaling amplifier 110. Multiplexer 108includes a first column of switches 154 having switch inputs forreceiving sense signals 130. Multiplexer 108 may include any number ofswitch inputs for receiving any number of sense signals. Switches 154have respective switch positions controlled responsive to a controlsignal component se10 common to both control signals 126 and controlsignals 124. The outputs of switches 154 feed the inputs of a secondcolumn of switches 156 having respective switch positions controlledresponsive to a second control signal component se11 also common to bothcontrol signals 126 and control signals 124. The outputs of switches 156feed the inputs of an output switch 158 of multiplexer 158. Outputswitch 158 has a switch position controlled responsive to a controlsignal component se12 of control signal 124.

[0054] Scaling amplifier 110 scales selected sense signal 152 accordingto a dynamic range of digitizer 112, to produce a scaled sense signal160. Digitizer 112 digitizes scaled sense signal 160 into a digitizedsense signal 164 and provides the digitized sense signal to controller114. Typically, digitized signal 164 is represented as a digital code.Thus, digitized signal 164 is considered representative of scaled signal160, and also of selected signal 152.

[0055] Collectively, multiplexer 108, scaling amplifier 110, anddigitizer 112 represent a digitizer module 166. Digitizer module 166produces, responsive to one or more of sense signals 130, one or more ofdigitized signals or codes 164 representative of the one or more ofsense signals 130. Digitizer module 166 may have many alternativearrangements. For example, scaling amplifier 110 may be omitted. Also,multiplexer 108 may be omitted, and replaced with multiple digitizers(such as digitizer 112) arranged in parallel with each other, todigitize all of sense signals 130 in parallel, and so on.

[0056] Controller 114 may include digital circuits, analog circuits,firmware, software, or any combination thereof, as would be apparent toone having ordinary skill in the relevant art(s). For example,controller 114 may be a computer system including a digital controllercoupled with (i) a memory for storing data and software programs, and(ii) input/output interfaces coupled to digitizer module 166 and sensecircuit 106. The software programs would include one or more softwaremodules for implementing the methods of the present invention. Indifferent arrangements of the present invention, one or more ofcontroller 114, multiplexer 108, scaling amplifier 110 and digitizer 112may be on-chip or off-chip. For example, controller 114 may be on-chip.

[0057] In the arrangement depicted in FIG. 1, at any given time,controller 114 receives digitized signal 164 corresponding to only oneof sense signals 130. Over time, controller 114 asserts control signals124 and 126 such that different or successive ones of sense signals 130are digitized and presented to controller 114. Thus, digitizer 112presents to controller 114 digitized signals or codes representative ofeach of analog signal 130 over time. Based on each of these digitizedrepresentations of sense signals 130, controller 114 determines thecorresponding process-based circuit parameters, temperature and/or powersupply voltage of IC chip 103.

[0058] For example, controller 114 determines: a value of sheet-rho fora low sheet-rho or a high sheet-rho resistor constructed on IC chip 103;a value of a gate-to-source threshold voltage for a PMOS or an NMOStransistor constructed on the IC chip; a value of a transconductanceparameter (K) of a PMOS or an NMOS transistor constructed on the ICchip; a temperature of the IC chip, and/or a power supply voltage of theIC chip. Based on one or more of these process-based circuit parameters,the temperature and/or the power supply voltage of IC chip 103,controller 114 may then determine one or more further circuit parametersassociated with operational circuit 116, for example. In an embodiment,controller 114 may then assert configuration control signal 120 so as toconfigure operational circuit 116 responsive to the determined one ormore operational circuit parameters, determined temperature and/ordetermined supply voltage.

[0059]FIG. 1A is a high-level block diagram of process monitor system102 corresponding to that shown in FIG. 1. Process monitor system 102includes a process monitor module 182 at least partially constructed onIC chip 103 and coupled with controller 114. Process monitor module 182includes circuits 104, 106, 108, and 112, or alternative arrangementsand portions thereof, for example, which cooperate to produce one ormore digitized sense signals 164 indicative of one or more ofprocess-dependent circuit parameters, and/or temperature, and/or powersupply voltage of IC chip 103. For example, module 182 produces adigitized signal 164 a indicative of a process-dependent circuitparameter of IC chip 103, a digitized signal 164 b indicative of atemperature of IC chip 103, and a digitized signal 164 c indicative of apower supply voltage of IC chip 103. Module 182 may provide each ofdigitized signals 164 to controller 114 in parallel (i.e., concurrently)or serially (i.e., sequentially). Controller 114 operates on digitizedsignals 164 and controls the operation of circuit 182.

[0060] III. Process Monitor Bias Circuit and Sense Circuit

[0061]FIG. 2 is a circuit diagram of an example arrangement of biascircuit 104 and process monitor sense circuit 106. Bias circuit 104 isrepresented above a dashed-line 202, while process monitor sense circuit106 is represented below the dashed line 202. Example circuitparameters, including current magnitudes and types, and transistordimensions, are indicated in FIG. 2. Bias circuit 104 is at leastpartially constructed on IC chip 103. Bias circuit 104 includes multiplebias current generators 204, 205, 206 and 207 for generating respectivebias currents 11, 12, 13, and 14. Each of bias generators 204-207receives power from a power supply rail 208 of IC chip 103, at a voltageVDD.

[0062] Bias current I1 is a CTAT bias current referenced to an externalresistor. A CTAT bias current is a bias current that remains constant asthe temperature of IC chip 103 varies. The CTAT bias current is alsorelatively constant as power supply voltage (VDD) varies. The CTAT biascurrent is also constant with respect to IC chip process variations,because the current is referenced to an off-chip resistor, as isdiscussed below in connection with FIGS. 3 and 4. Another reason theCTAT bias current is constant with respect to process is that the biascurrent is generated based on a bandgap voltage (described below) thatdoes not vary with process. Bias current I1 has an example, relativelyhigh current magnitude of 1.2 milli-Amperes (mA). Similarly, biascurrent I2 is a CTAT bias current referenced to an external resistor andhas an example, relatively low current magnitude of 200 uA. The externalresistor can be independent of process, temperature and VDD variations.

[0063] Bias current I3 is a PTAT bias current referenced to a poly-highsheet-rho resistor, having an example current magnitude of 200 uA.Current 13 varies in proportion to the absolute temperature (in Kelvin)of IC chip 103. However, the PTAT bias currents in the present inventionare essentially independent to VDD variations. Current I3 can vary witha poly-high sheet-rho of a resistor used in generating the current.

[0064] Bias current I4 is a CTAT bias current referenced to poly-highsheet-rho resistor, having an example current magnitude of approximately200 uA.

[0065] Sense circuit 106 includes one or more individual sense elementcircuits 209 constructed on IC chip 103. Each of these sense elementcircuits is also referred to herein equivalently and interchangeably asa sense element, a monitor, or a sensor. Multiple sense elements 209include an NMOS k-Monitor 210, a PMOS k-Monitor 212, an NMOS Vt-Monitor214, a PMOS Vt-Monitor 216, a poly-low sheet-rho sense element 218, apoly-high sheet-rho sense element 220, a temperature sensor 222, and apower supply voltage (VDD) sensor 250 (depicted in FIG. 2A).

[0066] Sense circuit 106 also includes a switch circuit 224, includingmultiple bias current switches S1, S2, S3, S4 and S5, coupled betweenbias circuit 104 and sense elements 209. Switches S1-S5 may betransmission-gate switches. Switches S1-S5 selectively direct biascurrents 11, 12, 13 and 14 to selected ones, or a subset, of senseelements 209, responsive to switch control component signals se10 (alsoreferred to as bit0) and se11 (also referred to as bit1). Morespecifically, switch S1 directs bias current I1 to either sense element210 or sense element 212 in accordance with component signal se10.Collectively, switches S3, and S2 and S4, direct bias current I2 to oneof sense elements 214, 216, 218 and 220, in accordance with componentswitch control signals se10 and se11. Also, switch S5 directs eitherbias current I3 or bias current I4 to sense element 222 in accordancewith component signal se10.

[0067] Although sensor 250 is depicted in FIG. 2A as having a dedicatedcurrent source (260), in an alternative arrangement, any of currentsources 205 and 207 may be switched to supply a CTAT bias current tosense element 250.

[0068] A. Transconductance Parameter (K) Monitors

[0069] Sense element 210 includes a relatively small NMOS transistor M1connected or configured as a diode. That is, transistor M1 includes agate and a drain connected together at a node 228, and a sourceconnected to a ground terminal 229 of IC chip 103, that is at a groundpotential (GND). Transistor M1 is a composite transistor. A compositetransistor includes a plurality of unit transistors connected inparallel with each other. Such composite transistors are known. Assumethe composite transistor is constructed using m unit transistors. Then,the composite transistor is an m gate device. The gate has a unit width,such as 5 microns (5 um), for example. The composite or effective widthW (sometimes referred to as We in the literature) is equal to m timesthe unit width. Also, the composite transistor has a length L, such as0.35 microns (0.35 um), depending the CMOS process. The transistorsdepicted in FIG. 2 are composite transistors. In FIG. 2, the descriptor“10/0.35,” near the reference identifier M1, indicates transistor M1 hasexemplary characteristics “We/L,” where We=10 um and L=0.35 um.

[0070] Sense element 210 optionally includes a potential voltage dividerincluding a resistor R1 and a resistor R2 connected in series with oneanother and between node 228 and ground. Sense signal 142 is tapped-offfrom a node 230 between resistors R1 and R2. The voltage dividerestablishes sense signal 142 at a voltage commensurate with a dynamicrange of digitizer 112. The use of large resistance value resistors inthe voltage divider limits current through the resistors, which reduceserror in the sense voltage. In another arrangement, sense signal 142 maybe taken directly from node 228. In another arrangement, a bufferamplifier having an exemplary gain of 0.5 can be used instead of aresistor voltage divider.

[0071] When switch S1 is positioned so as to direct or supply biascurrent I1 to sense element 210, transistor M1 develops a gate-to-sourcevoltage at node 228. Transistor M1 is sized in relation to the magnitude(ID) of current I1 so that transistor M1 operates at a relatively highcurrent density (HCD) JHCD. Thus, transistor M1 is referred to as a highcurrent density device. Due to the high current density in transistorM1, the gate-to-source voltage established at node 228 is substantiallygreater than the gate-to-source threshold voltage of the transistor. Forexample, in a 0.35 micron CMOS process, the gate-to-source voltage atnode 228 may be in the approximate voltage range of 1.5 to 2.0 volts,whereas the threshold voltage may be only approximately 500 millivolts.The difference between the voltage at node 228 and the gate-to-sourcethreshold voltage of transistor M1 is indicative of the transconductanceparameter (K) of the transistor. In this way, the voltage at node 228 isindicative of the transconductance parameter. Stated otherwise,transistor M1 is biased at a large value of gate-to-source voltage (VGS)minus threshold voltage (VTH), which provides useful information on thetransconductance parameter K. Because the voltage at node 228 isproportional to the voltage of sense signal 142 (nmos_k), sense signal142 is similarly indicative of the transconductance parameter.

[0072] Sense element 212 is similar in construction and operation tosense element 210 except that sense element 212 includes a relativelysmall PMOS transistor M2 connected as a diode, instead of the NMOStransistor used in circuit 210. Sense element 212 also includes anoptional potential divider, including resistors R3 and R4 connected inseries with each other and between node 230 and ground. Similar toabove, the voltage divider establishes sense signal 140 at a tap-pointbetween the resistors, such that the voltage is commensurate with adynamic range of digitizer 112.

[0073] When switch S1 directs relatively large bias current I1 to senseelement 212 as depicted in FIG. 2, transistor M2 develops agate-to-source voltage at a node 230 (which is connected to the sourceof transistor M2). Transistor M2 is sized in relation to the magnitudeof current I1 so that transistor M2 operates at a relatively highcurrent density JHCD. Thus, the voltage at node 230 is substantiallygreater than the gate-to-source threshold voltage of transistor M2. Thevoltage at node 230, and correspondingly, the voltage of sense signal140 (pmos_k), is indicative of the transconductance parameter oftransistor M2.

[0074] B. Threshold Voltage V_(TH) Monitors

[0075] Sense element 214 includes a relatively large NMOS transistor M3connected as a diode between ground and an output node 232 (which isconnected to the gate and drain of transistor M3). When switches S3 andS2 are configured to direct or supply relatively small bias current I2to sense element 214, transistor M3 develops a gate-to-source voltage atnode 232 (i.e., sense signal 138 (nmos_vt)) responsive to current 12.Transistor M3 is sized in relation to the magnitude of current I2 suchthat transistor M3 operates at a low current density (LCD) JLCD, e.g.,in relation to the high current density at which transistors M1 and M2operate. This low current density causes the gate-to-source voltagedeveloped at node 232 to be at or near the gate-to-source thresholdvoltage of transistor M3. For example, in a 0.35 micron CMOS process,the threshold voltage may be approximately 500 millivolts. Thus, sensesignal 138 is representative of the gate-to-source threshold voltage oftransistor M3.

[0076] Sense element 216 includes a relatively large PMOS transistor M4connected as a diode between ground and an output node 234 (which isconnected to source of transistor M4). When switches S3 and S2 areconfigured to direct or supply relatively small bias current I2 to senseelement 216, transistor M4 develops a gate-to-source voltage at node 234representing sense signal 136 (pmos_vt). Transistor M4 is sized inrelation to the magnitude of current I2 such that transistor M4 operatesat a low current density, e.g., in relation to the high current densityat which transistors M1 and M2 operate. This low current density causestransistor M4 to develop a voltage at node 232 that is at or near thegate-to-source threshold voltage of the transistor. Thus, sense signal136 is representative of the gate-to-source threshold voltage oftransistor M4.

[0077] C. Sheet-Rho Monitors

[0078] If a known, constant bias current is applied to an knownresistance value of an on-chip resistor, the voltage developed acrossthe resistor will be proportional to its resistance value. A voltagehigher than nominal would indicate high sheet resistance (sheet-rho) forthis type of resistor, and vice-versa. Sense element 218 and 220 takeadvantage of this effect.

[0079] Sense element 218 includes a resistor R5 connected between anoutput node 236 and ground. Resistor R5 is made of a resistancematerial, such as polysilicon, having a relatively low sheet-rho. Whenswitches S3 and S2 are configured to supply bias current I2 to resistorR5, the resistor develops a voltage at node 236 corresponding to sensesignal 134 (pl_rho).

[0080] Sense element 220 includes a resistor R6 coupled between groundand output node 238. Resistor R6 is made of a resistance material suchas polysilicon and has a relatively high sheet-rho, compared to thesheet resistance of resistor R5. When switches S3 and S4 direct biascurrent I2 to resistor R6, the resistor develops a voltage at outputnode 238 corresponding to sense signal 132 (ph_rho).

[0081] D. Temperature Sensor

[0082] Sense element 222 includes a resistor R7 connected between anoutput node 242 and ground. When switch S5 is configured to direct CTATbias current I4 to resistor R7, resistor R7 develops a first voltage atnode 242. On the other hand, when switch S5 is configured to direct PTATbias current I3 to resistor R7, the resistor develops a second voltageat node 242. A difference between the first voltage and the secondvoltage developed at node 242 indicative of a temperature of IC chip103.

[0083] E. Power Supply Voltage (VDD) Sensor

[0084]FIG. 2A is a circuit diagram of sense circuit 250 for monitoringpower supply voltage VDD of IC chip 103. Sense circuit 250 is coupled toa power supply rail of IC chip 103. A voltage divider, includingresistors 254 and 256, presents a voltage V1 equal to one-half VDD to apositive input of an operational amplifier (opamp) 258. For example, ifVDD=3.3 Volts (V) and can vary +/−0.5V, then the positive input to opamp258 will be 1.65V+/−0.25V. Note that is not important what type ofresistors are used, so long as both resistors 254 and 256 match eachother.

[0085] A current source 260 supplies a CTAT current 262 referenced to aninternal (i.e., on-chip) poly-high resistor (see FIG. 3 for details).When current 262 drives a resistor 270, a Thevenin equivalent circuit isformed that is the same as a 2.3V fixed Direct Current (DC) voltagesource in series with a 10 kilo-ohm poly-high resistor. Since poly-highresistors are used in both the bandgap-based current reference (FIG. 3)and VDD sense element 250, variations in resistor sheet-rho arecancelled out. Resistor 270 sets the proper gain for VDD sense circuit250. The end result is a circuit with the following transfer function:

VDD _(—) SENSE=(2*V1)−2.3

[0086] where VDD_SENSE (signal 145) is indicative of power supplyvoltage VDD.

[0087] The transfer function above creates VDD_SENSE such that it rangesfrom 0.5V to 1.5V as VDD changes from 3.3-0.5V to 3.3+0.5V,substantially perfectly matching the input range of digitizer 112. Thus,VDD sense circuit 250 presents an input to digitizer 112 with fullresolution as VDD changes over a +/−0.5V range, from its nominal valueof 3.3V. Other VDD sense circuits can be designed based on similarapproaches.

[0088] F. Nominal Voltage Monitor

[0089] In an embodiment, process monitor sense circuit 106 includesanother type of monitor or sense circuit referred to herein as a“nominal voltage” monitor, depicted in FIG. 2B. FIG. 2B is a circuitdiagram of an example “nominal voltage” monitor 274. Monitor 274 has acircuit structure that is similar to or the same as that of the Kmonitor (e.g., monitor 210 or 212) or the VTH monitor (e.g., monitor 214or 216). For example, “nominal voltage” monitor 274 includes atransistor 275 configured to produce a sense signal (i.e., itsgate-source voltage) 276 responsive to a bias current 277 supplied tothe transistor. As will be described below, sense signal 276 isrepresentative of a nominal gate-to-source voltage, which is aprocess-dependant circuit parameter. “Nominal voltage” monitor 274provides sense signal 276 (i.e., its gate-to-source voltage) tomultiplexer 108, for example, in signal set 130.

[0090] Transistor 275 is configured similar to or in the same way as anyof transistors M1, M2 M3, or M4 in FIG. 2 are configured in theirrespective monitor circuits. However, in contrast to the K and VTHmonitors, transistor 275 is sized in relation to bias current 277 so asto operate at (i) a nominal current density that is between therelatively high and low current densities of the K and VTH monitors, and(ii) a corresponding nominal gate-to-source voltage that is between therelatively high and low gate-to-source voltages associated with the Kand VTH monitors. The nominal current density and corresponding nominalgate-to-source voltage are representative of a nominal (i.e., typical)current density and a corresponding nominal gate-to-source voltage,respectively, associated with transistors in operational circuit 116.For a 0.35 micron CMOS process, an example nominal gate-to-sourcevoltage is approximately 1.2 volts.

[0091] The example process-dependent circuit parameters described above,including (i) the nominal gate-to-source voltage, (ii) thetransconductance parameter (K), and (iii) the gate-to-source thresholdvoltage VTH, are together generally referred to as“transistor-dependent” process-dependent circuit parameters because theyrelate to (i.e., are asssociated with) and/or characterize transistorsconstructed on the IC chip. The present invention is not limited tothese transistor-dependent circuit parameters.

[0092] IV. Process Monitor Bias Circuit

[0093]FIG. 3 is a circuit diagram of an example arrangement of biascircuit 104. Bias circuit 104 includes a bandgap voltage referencecircuit 302 that generates a CTAT voltage 304 (V_CTAT), and a PTATvoltage 306 (V_PTAT). Circuit 302 provides CTAT voltage 304 to a firstcurrent generator 310 for generating CTAT bias current I1 (or forgenerating CTAT bias current I2) and a current generator 314 forgenerating CTAT bias current 14. Bandgap circuit 302 provides PTATvoltage 306 to a current generator 318 for generating PTAT bias currentI3.

[0094] Current generator 310 includes a circuit 320 for converting CTATvoltage 304 to a corresponding CTAT current 322. Circuit 320 includes anoperational amplifier 324 that receives CTAT voltage 304, and inresponse, drives the gate of a transistor 326. Transistor 326 has asource-drain current path connected in series with an off-chip resistor328 (R_ext). Resistor 328 is selected to have a very low temperaturecoefficient. Resistance 328 is also selected to have a tight toleranceof between 1% and 5% in its absolute resistance value, typically.Current generator 310 also includes a current mirror 330 coupled betweencircuit 320 and a power supply rail at the voltage VDD. Current mirror330 includes a diode connected transistor 334 connected between thesource-drain path of transistor 326 and the power supply rail. Currentmirror 330 also includes a mirror transistor 336 coupled to transistor334. Current mirror 330 mirrors current 322 in output transistor 336, aseither bias current I1 (or bias current I2). Transistors 334 and 336 maybe sized in relation to each other to scale the magnitude of current I1(or I2) in relation to the magnitude of current 322.

[0095] Current generator 314 includes a circuit 350 (similar to circuit320), coupled with a current mirror 352 (similar to current mirror 330).Circuit 350 include a resistor 356 made of a resistance material havinga relatively high sheet-rho. Preferably, resistor 356 is constructedon-chip so as to match other on-chip resistors.

[0096] Current generator 318 is configured in the same manner as currentgenerator 314. However, current generator 318 generates bias current I3from PTAT voltage 306. Thus, bias current I3 is a PTAT bias currentinstead of a CTAT bias current. Alternative circuits may be used forgenerating the PTAT and CTAT currents used in the present invention, aswould be apparent to one having ordinary skill in the relevant art(s)given the present description.

[0097]FIG. 4 is a circuit diagram of an arrangement of a bandgap voltagereference circuit 302. Circuit 302 includes a pair diode-connected ofbipolar transistors Q1 and Q2 connected with a network of resistors R10,R11 and R12. Tap points off of the resistor network feed an operationalamplifier 404 that generates CTAT voltage 304 (also referred to as VREFin FIG. 4). Circuit 302 produces PTAT voltage 306, or a voltage fromwhich the PTAT voltage may be derived, at a node 406 of the resistornetwork, possibly with the aid of a scaling amplifier.

[0098] V. Reduced Complexity Process Monitor System

[0099]FIG. 5 is a block diagram of a simplified arrangement of a processmonitor, according to an embodiment of the present invention. Processmonitor system 500 includes a bias circuit 502 that generates a biascurrent 504. Bias circuit 502 includes one or more of current generatorsI1-I5. A sense element 506 produces a sense signal or voltage 508responsive to bias current 504. Sense element 506 may be any one ofsense circuits 210-222, 250 and 270, for example. Digitizer 112digitizes sense signal 508 into digitized code 164. In the simplifiedprocess monitor, digitizer module 166 reduces to digitizer 112.Digitizer 112 may be implemented in any number of ways. For example,digitizer 112 may include a digital-to-analog converter (DAC) and alatching comparator each coupled to controller 114. The comparatorcompares a received sense voltage against an analog voltage from theDAC, which is established by controller 114. The controller “searches”through a set of analog voltages from the DAC using a successiveapproximation register (SAR) routine, to determine a digital value ofthe received sense voltage. Such an implementation would be apparent toone having ordinary skill in the relevant art(s).

[0100] Controller 114 converts digitized code 164 into a correspondingvalue of the parameter sensed by sense element 506.

[0101] VI. Processing Sense Voltages and Controller Methods

[0102] In the ensuing mathematical analysis and description ofprocessing methods, each of the sense signals 130, and selected sensesignal 152, are represented generally as a voltage V_(sense), unlessotherwise indicated. Also, scaled signal 160 is represented generally asa voltage VSA, unless otherwise indicated. Also, digitized signal 164 isrepresented generally as “code.” Also, portions of the mathematicalanalysis below relate to excerpts from the following texts:

[0103] Phillip E. Allen & Douglas R. Holbert, CMOS Analog CircuitDesign, pp. 58-66 and 76-86, Saunders College Publishing 1987, ISBN0-03-006587-9; and

[0104] David A. Johns & Ken Martin, Analog Integrated Circuit Design,pp. 16-39 and 57-60, John Wiley & Sons, Inc. 1997, ISBN 0-471-14448-7,each of the excerpts being incorporated herein in its entirety byreference.

[0105] A. Converting Sense Voltage to Digitizer Code

[0106] Digitizer 112 converts scaled sense signal 160 to digital code164. In an example arrangement, digitizer 112 produces a 6-bit codehaving a range corresponding to a range of scaled sense signal 160.Digitizer 112 digitizes voltages between 0.5 and 1.5 volts, with 6-bitresolution. Voltages below 0.5 volts result in a code of 0 and voltagesabove 1.5 volts result in a code of 63. A resolution of 6-bits is givenonly as an example, and any other resolution may be used. Digitizer 112produces a code from an input voltage according to the followingcode-to-scaled-sense-voltage relationship: $\begin{matrix}{{code} = {{int}\left\lbrack {\frac{V_{SA} - 0.5}{1.0} \cdot (63)} \right\rbrack}} & {{eq}.\quad (1)}\end{matrix}$

[0107] where: VSA represents a scaled sense voltage (i.e., the magnitudeof scaled signal 160); and

[0108] “code” represents the code value produced by digitizer 112,responsive to scaled sense voltage VSA.

[0109] For example, let VSA=1050 mV, then:

code=int[34.65]=34

[0110] B. Converting Digitizer Code to Scaled Sense Voltage

[0111] Controller 114 can determine a voltage level (or “voltage”) ofscaled sense signal 160 based on the value of digitized code 164. Forexample, controller 114 can convert a digitizer code to a correspondingscaled sense voltage, according to the followingscaled-sense-voltage-to-code relationship: $\begin{matrix}{V_{SA} = {\frac{{code} \cdot (1.0)}{63} + 0.5}} & {{eq}.\quad (2)}\end{matrix}$

[0112] For example, let code 23, then:$V_{SA} = {{\frac{27 \cdot (1.0)}{63} + 0.5} = 0.9286}$

[0113] C. Converting Scaled Voltage to Sense voltage

[0114] Controller 114 can use the following relationships to convert ascaled sense voltage (i.e., the magnitude of scaled sense signal 160)into a sense voltage (i.e., the magnitude of selected signal 152representative of one of sense signals 130):

V _(SA)=1.667·V _(sense)−0.1667

[0115] where Vsense represents the voltage level of selected sensesignal 152 (and thus, of the selected one of sense signals 130).

[0116] To get the sense voltage Vsense: $\begin{matrix}{V_{sense} = \frac{V_{SA} + 0.1667}{1.667}} & {{eq}.\quad (3)}\end{matrix}$

[0117] For example, let VSA=1.000, then:$V_{sense} = {\frac{1.000 + 0.1667}{1.667} = 0.6999}$

[0118] The above relationships for converting scaled voltage VSA (signal160) to sense voltage V_(sense) (signal 152) take into account atransfer function of scaling amplifier 110.

[0119] D. Determining Temperature (T)

[0120] Controller 114 may determine the temperature of IC chip 103 basedon sense signal 144, as represented in digitized code 164. Thetemperature may be determined as follows. First, aprioritemperature-sense voltage data is established. For example, simulationsand/or actual test data may be used to find sense voltages of sensesignal 144 (therm) at different IC chip temperatures. Based on thisestablished apriori data, during the operation of process monitor 100,digitized code 164 can be used to determine IC chip temperature.

[0121] For example, consider the following apriori data:

[0122] When T=OC, V_(sense)=0.6370 volts

[0123] When T=100C, V_(sense)=0.8843 volts

[0124] where V_(sense) represents the voltage of sense signal 144.

[0125] Fitting these data points to a straight line:

V _(sense)=(2.473·10⁻³)·T+0.6370

[0126] Combining the previous equations (1), (2) and (3), and solvingfor T:

T=(3.851)·code−95.84  eq. (4)

[0127] For example, let code=33, then:

T=(3.851)·(33)−95.84=31.2

[0128] In another embodiment, temperature T may be determined based ondetermined values of sheet-rho and a bias current resistor size, and atemperature coefficient of the resistor.

[0129] E. Determining Transistor Threshold Voltage V_(TH)

[0130] Process monitor 102 can be used to determine a gate-to-sourcethreshold voltage of a transistor constructed on IC chip 103.Specifically, process monitor 102 can determine the gate-to-sourcethreshold voltage of either of transistors M3 and M4 in respective senseelements 214 and 216. The determined gate-to-source threshold voltage isrepresentative of the gate-to-source threshold voltages of othertransistors constructed on IC chip 103, for example, in operationalcircuit 116, because all such circuits use the same process.

[0131] The current I_(D) flowing in low current density (LCD) transistorM3 or M4, for example, is given by the following expression:$\begin{matrix}{I_{D} = {\frac{K}{2} \cdot \frac{W}{L} \cdot \left( {V_{GS\_ LCD} - V_{TH}} \right)^{2} \cdot \left\lbrack {1 - {\lambda \left( {V_{DS} - V_{eff}} \right)}} \right\rbrack}} & {{eq}.\quad (5)}\end{matrix}$

[0132] where:

[0133] I_(D) is the Direct Current (DC) drain current of the low currentdensity (LCD) transistor M3 or M4;

[0134] K is the transconductance parameter of the transistor, sometimesgiven as μ_(n)C_(ox) (where u_(n) is a mobility of electrons near asurface of the IC chip material, in units of meters² Volt-Second; andC_(ox) is a gate capacitance/unit area, in units picofarad/meter²);

[0135] W is the width of the transistor;

[0136] L is the channel length of the transistor;

[0137] V_(GS) _(—) _(LCD) is the gate-source voltage of the transistor.Here, V_(GS) _(—) _(LCD) corresponds to sense signal 138 (nmos_vt) or136 (pmos_vt);

[0138] V_(TH) is the threshold voltage of the transistor;

[0139] λ is the channel length modulation parameter (usually small);

[0140] V_(DS) is the drain-source voltage of the transistor; and

[0141] V_(eff) is the voltage at which the transistor is in-betweensaturation and triode operation.

[0142] Neglecting channel length modulation effects for mathematicalconvenience:$I_{D} = {\frac{K}{2} \cdot \frac{W}{L} \cdot \left( {V_{GS\_ LCD} - V_{TH}} \right)^{2}}$

[0143] Solving for V_(TH): $\begin{matrix}\begin{matrix}{V_{TH} = {V_{GS\_ LCD} - \sqrt{\left( \frac{2I_{D}}{K} \right) \cdot \left( \frac{L}{W} \right)}}} \\{\approx {V_{GS\_ LCD}\quad \left( {{for}\quad {very}\quad {low}\quad {drain}\quad {currents}} \right)}}\end{matrix} & {{eq}.\quad (6)}\end{matrix}$

[0144] (for very low drain currents) eq. (6)

[0145] Thus, if I_(D) is very low for the particular size device, thenV_(TH)=V_(GS) (where V_(GS) is V_(GS) _(—) _(LCD)). It is to beunderstood that channel length modulation effects need not be neglectedin the analysis above and below.

[0146] F. Determining Transistor Transconductance Parameter K

[0147] Process monitor 102 can be used to determine a transconductanceparameter K of a transistor constructed on IC chip 103. Specifically,process monitor 102 can determine the transconductance parameter K ofeither of transistors M1 and M2 in respective sense elements 210 and212. The determined transconductance parameter is representative of thetransconductance parameter of other transistors constructed on IC chip103, such as transistors M3 or M4, or transistors in operational circuit116.

[0148] The current I_(D) flowing through high current density transistorM1 or M2, for example, is given by: $\begin{matrix}{I_{D} = {\frac{K}{2} \cdot \frac{W}{L} \cdot \left( {V_{GS\_ HCD} - V_{TH}} \right)^{2} \cdot \left\lbrack {1 - {\lambda \cdot \left( {V_{DS} - V_{eff}} \right)}} \right\rbrack}} & {{eq}.\quad (7)}\end{matrix}$

[0149] where:

[0150] I_(D), K, W, L, V_(TH), V_(DS), and V_(eff) are as before, butcorrespond to transistors M3 and M4;

[0151] V_(GS) _(—) _(HCD) is the gate-source voltage of the high currentdensity transistor M1 or M2 in sense element 210 or 212. Here, V_(GS)_(—) _(HCD) corresponds to sense signal 142 (nmos_vt) or 140 (pmos_k),as adjusted by the resistive voltage divider (R1,R2, or R3,R4).

[0152] Neglecting channel length modulation effects for mathematicalconvenience:$I_{D} = {\frac{K}{2} \cdot \frac{W}{L} \cdot \left( {V_{GS\_ HCD} - V_{TH}} \right)^{2}}$

[0153] Solving for K: $\begin{matrix}{K = {\left( {2I_{D}} \right) \cdot \left( \frac{L}{W} \right) \cdot \frac{1}{\left( {V_{GS\_ HCD} - V_{TH}} \right)^{2}}}} & {{eq}.\quad (8)}\end{matrix}$

[0154] In the above expression for K, I_(D) is predetermined, that is itis known from accurate design of current mirrors in the bias circuitthat supply bias currents to the sense elements. W and L arepredetermined, that is, known from the design layout. V_(TH) can bedetermined as described above based on sense signals 138 (nmos_vt) or136 (pmos_vt). VGS_HCD is the sense voltage from transistor M1 or M2, insense element 214 or 216. Thus, K can be determined.

[0155] Thus, process monitor 102 determines the transconductanceparameter K in a two step process. First, process monitor 102 determinesthe gate-to-source threshold voltage V_(TH) of a transistor constructedon IC chip 103, based on sense signal 138 or 136. Next, process monitor102 determines the transconductance parameter K of a transistorconstructed on IC chip 103 based on the determined threshold voltageV_(TH) and sense signal 142 or 140.

[0156] G. Example of Determining V_(TH) and K

[0157] Let code_(LCD)=10, where code_(LCD) represents the digitized codecorresponding to sense signal 138 or 136;

[0158] Let code_(HCD)=34, where code_(HCD) represents the digitized codecorresponding to sense signal 142 or 140.

[0159] Assume that the high current density transistor M1 or M2 operatesat 1.2 mA.

[0160] First, we find VTH. Assuming that the low current density deviceM3 or M4 operates at VTH (due its very low current density), we find:$V_{sense\_ LCD} = {{V_{GS\_ LCD} \approx V_{TH}} = {{\frac{{code}_{LCD}}{105} + 0.4} = {{\frac{10}{105} + 0.4} = 0.495}}}$

[0161] where Vsense_LCD is the sense voltage produced by device M3 orM4.

[0162] Next, we find K. We convert the codeHCD for the high currentdensity device M1 or M2 to its corresponding sense voltage:$V_{sense\_ HCD} = {V_{GS\_ HCD} = {{\frac{{code}_{HCD}}{105} + 0.4} = {{\frac{34}{105} + 0.4} = 0.724}}}$

[0163] From the two sense voltages V_(sense) _(—) _(HCD) and V_(sense)_(—) _(LCD) (where V_(sense) _(—) _(LCD) corresponds to V_(TH)), we canfind the transconductance parameter, K:$K = {{\left( {2I_{D}} \right) \cdot \left( \frac{L}{W} \right) \cdot \frac{1}{\left( {V_{GS\_ HCD} - V_{TH}} \right)^{2}}} = {{{(2) \cdot \left( {1.2\quad {mA}} \right) \cdot \left( \frac{0.35\quad {um}}{2.5\quad {um}} \right)}\frac{1}{\left( {0.724 - 0.495} \right)^{2}}} = {{1.602 \cdot 10^{- 3}}\quad {A/V^{2}}}}}$

[0164] H. Example of Determining Sheet-Rho

[0165] Assume that

[0166] When R′=0.8, V_(sense)=0.6035 volts, and

[0167] When R′=1.2, V_(sense)=0.9420 volts

[0168] where: R′ is R_(actual)/R_(idea1), or a corresponding ratio ofsheet-rhos;

[0169] V_(sense) represents the voltage of sense signal 134 or 132.

[0170] Fitting these data points to a straight line:

V _(sense)=(0.8463)·R′−0.0735

[0171] Combining the previous equations (1) and (3) and solving for R′:$\begin{matrix}{R = {\frac{code}{88.57} + (0.5614)}} & {{eq}.\quad (9)}\end{matrix}$

[0172] where: “code” represents the digitized code (signal 164)corresponding to sense signal 134 or 132.

[0173] For example, let code=48, then:$R^{\prime} = {{\frac{48}{88.57} + (0.5614)} = 1.103}$

[0174] Thus with a code of 51, the poly-high sheet-rho is 10.3% high.

[0175] If the nominal value of R was 50 ohms, then:

R _(actual)=(50)·(1.103)=55.17 ohms

[0176] I. Determining Small-Signal Transconductance gm

[0177] Process monitor 102 can be used to determine the small-signaltransconductance gm of a transistor constructed on IC chip 103 (e.g., inone of sense elements 210-216, or in operational circuit 116). Thetransconductance gm of the transistor is given by the following generalexpression: $\begin{matrix}{g_{m} = \sqrt{2{{K\left( \frac{W}{L} \right)} \cdot {I_{D}}}}} & {{eq}.\quad (10)}\end{matrix}$

[0178] where:

[0179] g_(m) is the small-signal transconductance of the transistor;

[0180] K is the transconductance parameter;

[0181] W is the width of the transistor;

[0182] L is the channel length of the transistor; and

[0183] I_(D) is the drain current.

[0184] Assume W, L and Id have predetermined or known values. Then,g_(m) can be determined once K is determined. In an example, assume acomposite transistor is constructed on IC chip. The composite transistoris constructed using m=32 unit transistors. Then, the compositetransistor is a thirty-two (32) gate device, where each gate has a unitwidth of 5 um. Also, assume the composite transistor is a 0.35 microndevice, that is, that the length L=0.35 um. Also, assume that thecomposite transistor operates at an drain current of I_(D)=16 mA. Fromthese parameters and a determined value of K from above, we find thesmall signal transconductance gm of the composite transistor as follows:$g_{m} = {\sqrt{2 \cdot \left( {1.602 \cdot 10^{- 3}} \right) \cdot \left( \frac{32.5{\quad \quad}u\quad m}{0.35\quad {um}} \right) \cdot \left( {16\quad {mA}} \right)} = {{0.153\quad S} = {153\quad {mS}}}}$

[0185] The examples given above apply to CMOS processes. Similarcircuits and analysis can be applied to other processes. For example, ina Silicon-Germanium (SiGe) or other bipolar process, circuits andmethods along the lines of those described above may be used tosense/determine/analyze a base-to-emitter voltage and a collectorcurrent instead a gate-to-source voltage and a drain current.

[0186] VII. Example Operational Circuit

[0187] As mentioned above, process monitor 102 can be used to determineone or more process-based circuit parameters of IC chip 103. Theseprocess-based circuit parameters can then be used to determineoperational circuit parameters of operational circuit 116.

[0188]FIG. 6 is a circuit diagram of an exemplary amplifier 600,corresponding to operational circuit 116. Other types of circuits arecontemplated. Amplifier 600 includes an NMOS transistor 604, constructedon IC chip 103, having a source connected to ground, a gate connected toa voltage source 606, and a drain 608 that produces an output signalVout. Amplifier 600 includes a variable load resistor RL connectedbetween drain 608 and a power supply rail at voltage VDD. Load resistorRL may be varied in accordance with control signal 120.

[0189] A transistor-based amplifier having a resistive load RL, has avoltage gain A_(v) given by the following general expression:

A _(v) =g _(m) ·R _(L)  eq. (11)

[0190] For amplifier 600, g_(m) is the transconductance of transistor604, and RL is the resistance of load resistor RL. Here, it is assumedthat all resistors match in ratio on IC chip 103. Thus, resistor RLmatches resistor R5 or R6 of FIG. 2. Therefore, the value of resistor RLcan be determined from process monitor 102.

[0191] Using the values of (i) g_(m), and (ii) resistance determinedabove, the voltage gain of amplifier 600 is determined as

A _(v) =g _(m) ·R _(L)=(0.153)·(55.17)=8.441

[0192] Converting the voltage gain to units of dB:

A _(dB)=20·log₁₀(8.441)=18.53 dB

[0193] In example circuit 600, assume the nominal transconductance gm isequal to 130 milli-Siemens (where the unit Siemen=1/Ohm) and the nominalresistance is equal to 50 ohms. This gives a nominal voltage gain of 15dB. From our knowledge of K and sheet-Rho, we see that the gain is 3 dBhigher than nominal. circuit may be adjusted to return the gain to anominal value. Alternatively, a decision to not adjust the gain may bemade, and the determined gain of the circuit is recorded.

[0194] VIII. Method Flowcharts

[0195]FIG. 7 is a flowchart of an example method 700 that may beperformed by process monitor 102. A first step 702 includes generatingone or more bias currents. For example, bias circuit 104 generates oneor more bias currents 118.

[0196] A next step 704 includes producing, responsive to the one or morebias currents, one or more sense signals each indicative of acorresponding process-dependent circuit parameter of an IC chip. Forexample, sense circuit 106 produces, responsive to the one or more biascurrents 118, one or more sense signals 130 each indicative of acorresponding process-dependant circuit parameter of IC chip 103, atemperature of the IC chip, and a power supply voltage of the IC chip.Any subset of these sense signals may be produced in this step.

[0197] A next step 706 includes producing, responsive to the one or moresense signals, one or more digitized signals each representative of acorresponding one of the one or more sense signals. For example,digitizer module 166 produces digitized signal(s) 164 corresponding tosense signals 130.

[0198] Steps 702, 704 and 706 collectively represent a step of producingone or more digitized sense signals each representative of correspondingones of one or more process-dependent circuit parameters, a temperature,and/or a power supply voltage of IC chip 103. Step 702 is optional, andmay be omitted in an alternative arrangement of method 700. Processmonitor module 182 may perform step 710.

[0199] A next step 708 includes determining values of one or more of theprocess-dependant circuit parameters, the IC chip temperature, and thepower supply voltage of the IC chip, based on one or more of thedigitized signals. For example, controller 114 determines values of theprocess-dependant circuit parameters, the temperature, and the powersupply voltage based on digitized signal(s) 164.

[0200]FIG. 8 is a flowchart of further method steps performed in step706 of method 700. Step 706 includes a further step 802. Step 802includes selecting over time different sense signals among the one ormore sense signals from step 704. For example, multiplexer 108 selectsone of sense signals 130 responsive to control signals 124.

[0201] An optional next step 804 includes scaling the selected sensesignals to produce corresponding scaled sense signals. For example,scaling amplifier 110 can perform this step.

[0202] A next step 806 includes digitizing the scaled, selected sensesignals to produce corresponding ones of the one or more digitizedsignals. For example, digitizer 112 can perform this step.

[0203]FIG. 9 is a flowchart expanding on step 708 of method 700. A firststep 902 includes converting a digitized code into a corresponding sensevoltage of one of the sense signals. This may take into account scalingvoltage V_(SA). One or more of equations (1)-(3) may be used, forexample. For example, controller 114 converts digitized code 164 to acorresponding sense voltage V_(sense).

[0204] A next step 904 includes determining a value of a circuitparameter corresponding to the digitized code, based on the sensevoltage V_(sense). For example, controller 114 determines the value ofthe circuit parameter from the determined voltage V_(sense). One or moreof equations (4)-(10) may be used, for example. In addition oralternatively, a value of an IC chip temperature or power supply voltagemay be determined in this step.

[0205] Steps 902 and 904 may be repeated to determine a circuitparameter, such as transconductance parameter K, that depends on othercircuit parameters, such as gate-source threshold voltage V_(TH).

[0206] IX. Controller Modules

[0207]FIG. 10 is a block diagram of an example controller architecture1000 for performing various functions and methods associated withembodiments of the present invention described above. Controller 1000includes the following modules, which may be implemented in hardware,software, firmware, or a combination thereof:

[0208] a process module 1004 for determining values of process-dependentcircuit parameters based on digitized sense signals indicative of theprocess-dependent parameters. Process module 1004 may includesub-modules each for determining a corresponding one of theprocess-dependent parameters, e.g., the transconductance parameter, thegate-to-source voltage, the resistivity, and so on;

[0209] a temperature module 1006 for determining a temperature valuebased on a digitized sense signal indicative of temperature;

[0210] a supply voltage module 1008 for determining a power supplyvoltage value based on a digitized sense signal indicative of thevoltage;

[0211] an operational circuit control module 1010 forcontrolling/adjusting operational circuit 116 responsive to resultsproduced by the other modules depicted in FIG. 10; and

[0212] a control module 1012 for controlling process monitor 182, forexample.

[0213] These modules intercommunicate over an interface or bus 1020.

[0214] X. Advantages

[0215] The process monitor provides a means for determining absolutecircuit parameters, and/or a temperature the IC chip, and/or a powersupply voltage (VDD) of the IC chip. Individual circuit elements may beadjusted based on the absolute circuit parameters.

[0216] The process monitor can be used with any integrated circuitprocess, such as CMOS, Silicon bipolar, GaAs, SiGe and InP processes.

[0217] An external processor is used to determine circuit parametersinstead of relying on complicated analog correction circuits on theintegrated circuit chip. The external processor is well-controlled.

[0218] The process monitor also provides a way to monitor process,supply voltage (VDD), temperature and other parameters to determine howmanufacturing variations occur. Self-contained (i.e., on-chip) analogcorrection circuits provide no information to a manufacturing tester.Also, the process monitor provides digital words, which are more readilyusable with an automated tester. For example, an automated tester candetermine, from information received from the process monitor, whether aparticular IC chip being monitored will be unable to meet circuit designspecifications. An out-of-spec. IC chip can be discarded before othermore costly tests are performed. Such information allows a foundry to“tweak” the process used in integrated circuit fabrication for bestperformance.

[0219] The process monitor is especially attractive in CMOS processesbeing used to make mixed-mode analog/digital circuits. Digital CMOSdigital processes vary widely, but support complicated digital controlcircuitry. The use of the process monitor allows a designer to designsimpler analog circuits that contain switch-able or re-configurablecircuit elements that may be adjusted based on information provided bythe process monitors.

[0220] The process monitor may be turned off after it is used, to savepower. In contrast, analog correction circuits typically remain alwayson.

[0221] XI. Conclusion

[0222] While various embodiments of the present invention have beendescribed above, it should be understood that they have been presentedby way of example, and not limitation. It will be apparent to personsskilled in the relevant art that various changes in form and detail canbe made therein without departing from the spirit and scope of theinvention. For example, embodiments include a process monitor sensecircuit and methods corresponding thereto, other embodiments include aprocess monitor sense circuit in combination with a digitizer or adigitizer module and methods corresponding thereto, further embodimentsinclude a process monitor sense circuit in combination with a digitizermodule and a controller and methods corresponding thereto, even furtherembodiments include an apparatus that produces digitized sense signals,an apparatus that produces digitized sense signals and a controller forprocessing the same and methods corresponding thereto, other embodimentsinclude a process monitor module in combination with a controller andmethods corresponding thereto, other embodiments include methods ofdetermining values of process-dependent circuit parameters, atemperature, and a power supply voltage of an IC chip.

[0223] The present invention has been described above with the aid offunctional building blocks and method steps illustrating the performanceof specified functions and relationships thereof. The boundaries ofthese functional building blocks and method steps have been arbitrarilydefined herein for the convenience of the description. Alternateboundaries can be defined so long as the specified functions andrelationships thereof are appropriately performed. Any such alternateboundaries are thus within the scope and spirit of the claimedinvention. One skilled in the art will recognize that these functionalbuilding blocks and modules can be implemented by discrete componentsincluding digital and/or analog circuits, application specificintegrated circuits, processors executing appropriate software,hardware, firmware and the like or any combination thereof. Thus, thebreadth and scope of the present invention should not be limited by anyof the above-described exemplary embodiments, but should be defined onlyin accordance with the following claims and their equivalents.

What is claimed is:
 1. A method of monitoring an integrated circuitchip, comprising: (a) receiving at least one digitized sense signal fromthe integrated circuit chip, whereby the at least one digitized sensesignal represents a corresponding process-dependent parameter within theintegrated circuit chip; and (b) determining an analog value for the atleast one process-dependent circuit parameters from the corresponding atleast one digitized signal; wherein the process-dependent parameter ismeasured within a process monitor portion of the integrated circuit andthe at least one determined analog value is utilized to correct for theprocess-dependent parameter in an operational portion of the integratedcircuit.
 2. The method according to claim 1, wherein steps (a) and (b)are performed outside of the integrated circuit chip.
 3. The methodaccording to claim 1, wherein step (b) comprises retrieving the at leastone value from a look-up table using the at least one digitized signal.4. The method according to claim 1, wherein step (b) comprisescalculating the at least one value from the at least one digitizedsignal.
 5. The method according to claim 1, wherein the at least onedigitized sense signal represents a gate-to-source threshold voltage ofa transistor fabricated on the integrated circuit chip.
 6. The methodaccording to claim 1, wherein the at least one digitized sense signalrepresents a transconductance parameter of a transistor fabricated onthe integrated circuit chip.
 7. The method according to claim 1, whereinthe at least one digitized sense signal represents a sheet resistance ofa resistor fabricated on the integrated circuit chip.
 8. The methodaccording to claim 1, wherein the at least one digitized sense signalrepresents a temperature of the integrated circuit chip.
 9. The methodaccording to claim 1, wherein the at least one digitized sense signalrepresents a power supply voltage on the integrated circuit chip. 10.The method according to claim 1, wherein the at least one digitizedsense signal includes a plurality of digitized sense signals thatrepresent a plurality of the following: a gate-to-source thresholdvoltage of a transistor fabricated on the integrated circuit chip; atransconductance parameter of a transistor fabricated on the integratedcircuit chip; a sheet resistance of a resistor fabricated on theintegrated circuit chip; a temperature of the integrated circuit chip;and a power supply voltage on the integrated circuit chip.
 11. A systemfor monitoring an integrated circuit chip, comprising: means forreceiving at least one digitized sense signal from the integratedcircuit chip, whereby the at least one digitized sense signal representsa corresponding process-dependent parameter within the integratedcircuit chip; and means for determining an analog value for the at leastone process-dependent circuit parameters from the corresponding at leastone digitized signal; wherein the process-dependent parameter ismeasured within a process monitor portion of the integrated circuit andthe at least one determined analog value is utilized to correct for theprocess-dependent parameter in an operational portion of the integratedcircuit.
 12. The system according to claim 11, wherein the means forreceiving and the means for determining are positioned external of theintegrated circuit chip.
 13. The vaccording to claim 11, wherein themeans for determining comprises means for retrieving the at least onevalue from a look-up table using the at least one digitized signal. 14.The system according to claim 11, wherein the means for determiningcomprises means for calculating the at least one value from the at leastone digitized signal.
 15. The system according to claim 11, wherein theat least one digitized sense signal represents a gate-to-sourcethreshold voltage of a transistor fabricated on the integrated circuitchip.
 16. The system according to claim 11, wherein the at least onedigitized sense signal represents a transconductance parameter of atransistor fabricated on the integrated circuit chip.
 17. The systemaccording to claim 11, wherein the at least one digitized sense signalrepresents a sheet resistance of a resistor fabricated on the integratedcircuit chip.
 18. The system according to claim 11, wherein the at leastone digitized sense signal represents a temperature of the integratedcircuit chip.
 19. The system according to claim 11, wherein the at leastone digitized sense signal represents a power supply voltage on theintegrated circuit chip.
 20. The system according to claim 11, whereinthe at least one digitized sense signal includes a plurality ofdigitized sense signals that represent a plurality of the following: agate-to-source threshold voltage of a transistor fabricated on theintegrated circuit chip; a transconductance parameter of a transistorfabricated on the integrated circuit chip; a sheet resistance of aresistor fabricated on the integrated circuit chip; a temperature of theintegrated circuit chip; and a power supply voltage on the integratedcircuit chip.